As the number of users of wireless communication systems, such as cellular radio systems, increases and fast data transmission becomes more common in these systems, it is vitally important to increase the capacity of the systems by improving their performance. One way to solve this problem is to use one or more adaptive antenna arrays instead of sector antennas. In an antenna array individual antenna elements are typically located close to one another, i.e. at a distance corresponding to about half a wavelength. Such arrays typically comprise as many antennas as are needed to achieve the desired coverage area.
Since sampling in the frequency domain causes folding in the time domain, a signal is usually divided into N samples and a discrete Fourier transformation DFT is calculated. An FFT (Fast Fourier Transformation) is an algorithm for reducing the number of complex multiplications needed to calculate the DFT. A particularly efficient calculation algorithm is obtained when N is a power of two. To facilitate the Fourier transformation, an antenna array usually comprises a number of antennas that is divisible by 2. The DFT and the FFT are described more closely in Proakis and Manolakis: Introduction to Digital Signal Processing, pp. 682-730.
When adaptive antenna arrays are used, the basic principle is to use narrow antenna beams that are directed towards the desired receiver as directly as possible. The generally known methods used with adaptive antenna arrays can be divided into two main groups: radiation beams are directed towards the receiver, or the most suitable beam is selected from among several alternative beams. A suitable beam is selected for downlink transmission, or the beam is turned on the basis of the information received from the uplink. The reuse of frequencies can be increased and the power of transmitters reduced because the interferences caused for other users decrease thanks to directivity of the antenna beams.
In a digital system antenna beams are directed by dividing a signal into I and Q branches in baseband parts and by multiplying the signal of each antenna element by suitable weighting coefficients in a complex manner (phase and amplitude) and then adding the output signals of all antenna elements together. In this case the adaptive antenna array comprises, in addition to the antennas, a signal processor which automatically adapts antenna beams by means of a control algorithm by turning the antenna beams towards the signal that was the strongest of the measured signals. Directivity of the beams can also be implemented analogously by using fixed phasing circuits (Butler matrix) for generating orthogonal radiation beams, in which the phase increases antenna by antenna. According to the method, it is simply measured which beam receives the largest amount of signal energy, i.e. with which beam the signal is the strongest, and this beam is selected for transmission.
A method where a directed antenna beam is widened is also known. According to this method, the incidence angle of the directed antenna array is formed in the base station system on the basis of a signal received from the uplink employing a prior art estimation method. The base station system transmits a signal to a subscriber terminal in the direction of the angle of departure formed from the incidence angle. To determine the width of the antenna's main beam, a ratio is calculated, which describes the unbalance between the uplink traffic and the downlink traffic, i.e. the differences in the amount of traffic within a certain period. The method is intended for use mainly in transmission of packet traffic.
A disadvantage of the method described above is that it is impossible to influence the level of side beams and thus the interference caused to other antenna sectors. Furthermore, the extent to which the main beam can be modified is relatively limited.